Plasma cell tumors in humans most commonly occur as multiple myeloma, an incurable form of cancer. Biochemical lesions involved in the development of these tumors have been difficult to characterize and the most common biological association with this disease is an apparent critical role of Interleukin-6. Although IL-6 is one growth factor of importance in myeloma development, we have previously demonstrated a role for the Insulin-like growth factor receptor I (IGF-IR) signaling pathway in myeloma both in vitro and in vivo. Recent studies have focused on biochemical characterization of this signaling pathway. Stimulation with IGF-I ligand leads to activation of two cellular signaling cascades, the mitogen activated protein kinase (MAPK) and PI-3K pathways. Analysis of downstream elements in the PI-3K pathway revealed activation of Akt kinase leading to inhibition of processes causing normal cell death. Akt additionally activates GSK-3 beta and p70S6 kinase. A series of metabolic inhibitors have been used to determine the roles of these various element in tumor cell proliferation and in crosstalk with other pathways. Inhibition of the MAPK pathway results in an approximate 20% decrease in proliferation whereas inhibition of the PI-3K pathway causes an 80% reduction in proliferation. Interestingly, inhibition of PI-3K also effects the MAPK pathway indicating crosstalk between these two cascades but only in one direction as inhibition of MAPK activity does not effect PI-3K. Inhibition of p70S6 kinase decreases proliferation ~ 40% and activation of this kinase is blocked by both MAPK and PI-3K inhibitors. These results indicate extensive cross talk between these two pathways and the mechanisms regulating such interactions are currently under investigation. Wnt proteins have been shown to be critical elements regulating development and inappropriate expression of Wnts has been observed in human cancers. We have recently initiated studies to assess the potential role of Wnts in myeloma. Myeloma cells have been found to express mRNA encoding multiple Wnt receptors (Frizzled proteins) as well as the co-receptors, LDL receptor related proteins (LRP) 5 and 6. In contrast, B cell lymphomas representing an earlier stage in B cell differentiation demonstrate limited expression of Fz mRNA and do not express either of the LRP co-receptors. Exposure of myeloma cells to Wnt proteins results in activation of a classical Wnt/beta catenin pathway and transcriptional activation. Additionally, striking morphological changes are observed in myeloma cells concurrent with extensive rearrangement of the actin cytoskeleton. These morphological changes were found to be associated with activation of a second intracellular pathway involving the nucleotide exchange factor Rho and Rho-associated kinase. Current studies are in progress to further biochemically characterize Wnt signaling in myeloma and to determine the biological consequences linked to activation of Wnt associated signaling cascades. This project was formerly Z01 BC 05553-28 LG